Fix empty directory checks with wildcard find API

[Old-Ding]

Closes #66

Summary

• Update FF_isDirEmpty() so it enumerates the target directory contents directly.
• Avoid calling FF_FindFirst() for the emptiness check, because wildcard mode treats a path without a trailing separator as the directory entry itself.
• Preserve the documented FF_FindFirst() wildcard semantics for public callers.

Testing

• git diff --check upstream/main...HEAD
• Verified the touched file uses CRLF line endings and has no UTF-8 BOM.
• Not run: FreeRTOS+FAT unit tests, because this Windows host does not have cmake, Ruby, or a C compiler installed.